On-chip photonic circuits working at the single-quantum level play an important role for future quantum information processing [1]. Several approaches to assemble such quantum photonic devices from different fundamental photonic entities have been pursued [2]. For example, by using self-assembled quantum dots in semiconductor membranes, one can exploit the full power of semiconductor nanofabrication technology, and sophisticated structures have been demonstrated [1], even with on-demand coupling architectures [3]. However, this approach is limited to two-dimensional structures, except for few results obtained by extremely challenging manual membrane-stacking [4].
Another easy and low-cost way of fabricating photonic structures is optical lithography via direct laser writing (DLW) [5, 6] where a tightly focussed femtosecond laser beam is used to expose a photoresist. The use of multi-photon absorption enables a sequential 3D exposure by scanning the sample or the focus of the laser. For common negative-tone photoresists, unexposed parts are removed during a development step and the 3D polymer structures remain. DLW is well known for the fabrication of photonic crystals [6] or other photonic elements like resonators [7, 8] and waveguides [9]. In order to functionalise the structures with optically active material, fluorescent dyes [10], quantum dots [11] and metal nanoparticles [12] have been incorporated. However, until today there has been no 3D structure operating at the fundamental quantum level with single photons from single emitters being collected and routed. Moreover, no combinations of multiple optical elements (different resonators, couplers, waveguides) have been demonstrated. The reason is the lack of photostable quantum emitters which are compatible with the DLW process while still preserving the possibility for high-quality DLW fabrication.